A trio of new studies reshapes our understanding of Jupiter's moon Europa, calling into question the certainty of its famous water plumes, revealing a surprisingly quiet seafloor, and rewriting the formation story of the moon and its volcanic neighbor, Io.
Hubble Plume Evidence Downgraded
The statistical confidence in the existence of water vapor plumes on Europa has been significantly downgraded. A team led by the Southwest Research Institute (SwRI) reanalyzed 14 years of Hubble Space Telescope data on Lyman-alpha emissions from the moon.
According to a study published in Astronomy & Astrophysics, the reanalysis has reduced the confidence level from a previous high of 99.9% to less than 90%. The initial 2014 detection claim was found to have uncertainties, including the potential misplacement of Europa in Hubble images by a pixel or two, which could dramatically affect data interpretation.
Dr. Kurt Retherford (SwRI) stated that the evidence for water vapor plumes is "not as strong as first understood." Dr. Lorenz Roth (Royal Technical Institute, Sweden) added that the reduction in confidence is "insufficient to support the certainty of the original claims." This is a significant update, as water vapor plumes have been confirmed on other moons, such as Saturn's Enceladus and Jupiter's Io.
Seafloor Activity and Energy for Life
A separate modeling study suggests that Europa's global ocean seafloor is likely quiet and largely inactive, which may limit the chemical energy available to support life. The research identifies several factors contributing to this inactivity.
- Cooling History: Europa is smaller than Earth and is estimated to have cooled more rapidly. Most of its internal heat is thought to have dissipated billions of years ago, removing a consistent heat source to drive seafloor change.
- Weak Tidal Heating: While Jupiter's gravity generates tidal heating elsewhere in the system, Europa experiences gentler tides. Modern tidal stresses are considered insufficient to actively stir the seafloor, particularly at depths greater than 300 meters.
- Sealed Rocks: Even if fractures existed in the past, minerals could fill these gaps, sealing channels and blocking the circulation of ocean water through rock.
"The study notes that some chemical reactions may still occur in shallow rocky zones," and radiation-driven chemistry (radiolysis) could generate reactive compounds in Europa's rocky layer. Europa's ice shell also exhibits surface fractures that could open pathways for material exchange.
The study suggests that the upcoming Europa Clipper mission, scheduled to arrive in 2030, should focus its search for chemical energy in these shallow rocky areas or zones where the ice shell interacts with the ocean.
Formation History of Io and Europa
A third study indicates that the contrasting water content of Jupiter's moons Io and Europa was established during their formation. Io is known to be volcanically active and dry, while Europa is icy and possesses a subsurface ocean.
The research team tested two hypotheses: one suggesting that extreme conditions near Jupiter prevented water ice preservation during formation, and another proposing both moons formed with water but Io subsequently lost it. Using a numerical modeling framework that coupled internal thermal evolution with volatile escape, the team found that Io would not have been able to efficiently lose its water if it had formed with it, and Europa would have retained its water even under extreme conditions.
The findings support the conclusion that Io formed from dry materials and Europa from ice-rich building blocks, challenging previous assumptions that Io's high density resulted from a significant loss of volatiles after its formation. Future data from NASA's Europa Clipper and the European Space Agency's Juice mission, both scheduled to begin in 2031, are expected to provide further information on these conclusions.